1. Field of the Invention
The present invention is directed to a process for the preparation of aqueous polyurea dispersions which contain blocked isocyanate groups and chemically incorporated isocyanate-reactive groups, have improved stability in water and are prepared by the emulsion polymerization process, and to the aqueous polyurea dispersions prepared by this process.
2. Description of the Prior
Aqueous polyurethane dispersions are known and are generally prepared by reacting a polyisocyanate with a high molecular weight polyol and low molecular weight chain extender components including components which provide hydrophilicity to the polyurethane. In these known processes, the polyisocyanate is initially reacted with the high molecular weight polyol to form an NCO prepolymer either in the melt or in the presence of an organic solvent prior to the introduction-of water.
In U.S. Pat. No. 3,479,310 the NCO prepolymer is chain extended to form a linear polyurethane which is then mixed with an organic solvent prior to the addition of water. In U.S. Pat. Nos. 4,066,591 and 4,408,008 the NCO prepolymers are mixed with a solvent, dispersed in water and then chain extended with low molecular weight polyamines.
Accordingly, even though the chain extension step may be conducted in either the organic or the aqueous phase, the reaction between the polyisocyanate and the high molecular weight polyol is not conducted in the presence of water in either process. Further, in both processes organic solvents are necessary to reduce the viscosity of the fully reacted polyurethane or the NCO prepolymer prior to dispersing them in water.
An object of copending application, U.S. Ser. No. 07/677,011, filed Mar. 28, 1991 now U.S. Pat. No. 4,212,230, was to provide a method for preparing aqueous polymer dispersions which did not require the use of organic solvents to reduce the viscosity of fully reacted polyurethanes or NCO prepolymers and which could be conducted using readily available, simple mixing equipment. In accordance with this application an aqueous polyurea dispersion was prepared by emulsion polymerization, i.e., a polyisocyanate was dispersed in water and subsequently reacted with a high molecular weight polyamine. In this manner the need for solvents to disperse NCO prepolymer or fully reacted polyurethanes was avoided.
One of the disadvantages of this process is that the polyurea dispersions do not remain stable for more than a few hours. This becomes a problem if the dispersions cannot be prepared at the site where they will be used, which is often the case. This disadvantage was overcome in copending application, U.S. Ser. No. 07/877,686, by blending the high molecular weight polyamine with a low molecular weight polyamine containing an anionic group. The additional hydrophilicity of the anionic group improved the stability of the polyurea dispersions.
However, the aqueous polyurea dispersions of this latter copending application also suffer from a disadvantage in certain applications which require a reactive system such as the binding of fiberglass. In order to adequately bind fiberglass using existing processes and production equipment, it is necessary for a polyisocyanate polyaddition reaction to occur at the elevated temperatures employed for the binding operation. Because the aqueous polyurea dispersions of the copending applications are fully reacted, they are not suitable for binding fiberglass using established production methods.
Accordingly, it is an object of the present invention to provide reactive aqueous polyurea dispersions which are suitable for use as fiberglass binders and which possess improved storage stability and do not sacrifice any of the valuable properties provided by the aqueous polyurea dispersions described in the copending applications.
This object can be achieved in accordance with the present invention as set forth hereinafter.